First step bring your own mobile phone and buy Thai SIM in Thailand or both I mean buy both Thai SIM and Mobile Phone together.
The SIM card 2 types
Register SIM or Post paid Sim or subscription sim you pay at the end of the month or monthly paid.
A second is Prepaid SIM you have to pay first by buying a slip or card to refill or top up your money first before making any phone call.
so in my view for a tourist just buy a Prepaid SIM is a good option for you.
The prepaid card price is begin from 50 bath,100,200,300,500 bath where you and get it from the local shop and in the city.
Calling rate is about 1 Baht per minutes for most companies such as AIS ,DATC,TRUE ,
For data and internet package is begin from 100 baht up to 1000 baht depends on the options and speed and volume of data.
For 3G/4G internet is cover all part in Thailand and get more quality better than a couple years ago.
If you are looking for more detail available in English below are the Link to their official website in English version
If you want to buy a mobile phone like iPhone 7 or Samsung galaxy s7 I would recommend you to buy it from the official website above or at the official shop of them in local market ,supermarket or city in order to make sure that you will get the real and genuine products
Most of them offer cheaper price when buying a mobile phone set along with the subscription package.
in some companies also release new sale promotion by giving away free mobile phone set for people just only pay for a SIM and put the money for your SIM balance start from 100 bath or 200 bath,
the brand and the phone feature is a basic phone for making s simple calling and receiving call ,in some case also capable of 3G/4G data as well.